The present invention relates to associative memory cells as used in Content Addressable Memories (CAM's) and also to memory units including pluralities of such cells.
Associative memories are devices in which the storage locations are identified according to their contents, and therefore are frequently called Content Addressable Memories (CAM's). Such memories are to be distinguished from the more widely-used coordinate-addressable memories, such as Random Access Memories (RAM's), which are identified and addressed according to the location or address of the particular cell. Thus, a Content Addressable Memory (CAM) is generally defined as a device capable of holding information, comparing that information with some specified information, and indicating agreement or disagreement between the two. More particularly, the CAM accepts as input a data word and a mask word, searches all address locations simultaneously for a match between the unmasked bits of the data word and all the stored words, and outputs, to a TAG-register, a marker or tag which identified the words which match the input data word in the unmasked bit positions.
Processors using associative memories, or CAM's, are generally recognized as providing two major advantages, namely, speed of operation, and ease of programming, over processors using the conventional coordinate-addressable memories, such as RAM's. However, their main disadvantage is their need for larger numbers of elements to perform the required logical functions, which increases the cost and size of these memories. CAM's have been known as far back as 1956, and since then many efforts have been directed to lowering their manufacturing costs. However, the known designs still are of relatively high cost and large size. Moreover, many or all of the known devices fail to provide operations essential in associative processing, which thereby limits their instruction range; an example of such a limit in the instruction range of the known CAM's is the inability to WRITE masked-ON bits while leaving the remainder unchanged.